U.S. Immigration is a mystery to many Americans, even lawyers. A lot of people know about passports, and some may be familiar with visas (not the credit card), but beyond that their understanding kind of drops off.
Visas, in case you know someone who hasn't heard of them, are printed in passports. They provide the holder permission to enter a country for a particular purpose and period of time. In the U.S., immigrant visas (IVs) allow a person to request entry in order to live here permanently, as a noncitizen. Non-immigrant visas (NIVs) allow someone to seek admission to stay for a temporary period of time, a few weeks to a few years, for a specific purpose like as a visitor or tourist, student, employee, investor, performer, medical patient, crew member, and other specific reasons.
Americans enjoy the privilege of entering almost 75% world's 195 countries as visitors without needing a visa, which may explain why so many of us are unfamiliar with the process. Yet, citizens from just 21% of the world's countries can visit the U.S. without a visa. Can you guess which countries are among this 21%? Canada (that was too easy), Bermuda (yep), and 39 others in the visa waiver program. Reading this list is like reading an elite "who's who" of the world's power players.
U.S. immigration assumes, as a foundational principle, that anyone entering this country from abroad plans to stay here for good, especially those from the other 79% of "non-listers". If you are someone who just wants to visit or maybe work or study here for a few years, you must prove that you don't plan to move here permanently. And, not surprisingly, people from less wealthy nations generally face greater scrutiny and must do more to show they don't have "immigrant intent". (Unless of course they do have immigrant intent, in which case they must apply for an immigrant visa.)
So for people from outside of the U.S. wanting to come here temporarily (especially if they aren't coming from a "21% country" for a vacation) a U.S. visa stamped into their passport is key. The visa is like an entry ticket, but it does not actually guarantee their entry into the U.S. It simply gives the holder permission to come to the border and request admission into the U.S. This inspection process, at a U.S. "Port of Entry" (generally located at airports, seaports, or roadways), is where the final decision is made whether to allow that person to enter, or not.
How to get a visa is the stuff of many blogs, as there are 185 different types of U.S. visas, both immigrant and non-immigrant combined. There are also other ways to enter the U.S. legally, for example as a refugee, asylum seeker, through humanitarian or other forms of parole, but these subjects also are for future blogs.